Switches have been developed in the past that are responsive to fluctuations in the level of a vacuum. Such switches may be used to generate a signal indicative of the status of a machine or to actuate a machine based on the detected vacuum level. For instance, such switches are used in U.S. Pat. No. 4,266,905 entitled "Apparatus For Acquiring Workpieces From A Storage Bin Or The Like". The apparatus disclosed therein utilizes suction to lift and carry randomly positioned articles from a container. The apparatus creates a vacuum at the end of an arm adapted to contact the article. However, when the arm is not in contact with one of the articles, the vacuum in the arm is reduced through exposure to the atmosphere. When an article is contacted, the end of the arm is closed off to the atmosphere and the level of the vacuum is increased. The increase in the vacuum is detected by a vacuum sensitive switch which generates a signal indicative of the fact that the arm has contacted and is carrying an article. U.S. Pat. No. 3,933,388 entitled "Interlock Control System For A Fluid-Operated Hoist" is another example of vacuum sensitive switches utilized to generate a signal responsive to fluctuations in the level of a vacuum.
However, conventional vacuum responsive switches exhibit certain inherent limitations and ineffeciencies. Specifically, conventional vacuum sensitive switches are not sensitive to relatively low levels of pressure or to relatively small pressure fluctuations, which are commonly encountered. Conventional switches typically have a pressure sensitivity of at least 4-5" water, whereas a greater sensitivity is desirable in many situations. Further, it is frequently desirable to adjust the level of vacuum which will actuate the switch, which is difficult to accomplish with conventional vacuum sensitive switches.